The present invention generally relates to image reading and printing units, and more particularly to an image reading and printing unit which is mounted on a carriage of a serial printer and reads and prints images.
Recently, proposals have been made to mount an image sensor on a carriage which moves a printing head of a serial printer, so as to carry out functions of an image scanner. In other words, the carriage prints an image on a sheet that is transported, and reads the printed document. Hence, there are demands to improve the quality of the reading with respect to the transported document, and it is necessary to prevent focal error, change in magnification and the like caused by flapping or the like of the transported document.
FIGS.1A and 1B show the construction of a conceivable carriage mounted with an image sensor. FIG. 1A is a plan view generally showing a carriage part of a serial printer, and FIG.1B is a front view generally showing this carriage part.
A thermal serial printer 10 shown in FIGS. 1A and 1B includes a carriage 11, and a thermal head 12 and an image sensor 13 which are mounted on the carriage 11. In a case where a plain paper is used as the recording sheet, a ribbon cassette 14 is also mounted on the carriage 11. This ribbon cassette 14 accommodates a ribbon made up of a thermal transfer film. The carriage 11 is moved in a scan direction A or B which is perpendicular to a transport direction of the sheet by a driving part 16 under the guidance of a guide part 15. The recording sheet is transported at the time of printing, and the document is transported at the time of reading. In addition, a platen 17 is arranged along the scan direction to confront the thermal head 12.
The image sensor 13 is provided on the side of the ribbon cassette 14 and has an opening confronting the platen 17. The image sensor 13 is made up of a compact optical system including a light source 18, a lens 19 and a solid state image pickup (or reading) element 20 arranged on an optical axis of the lens 19. The light source 18 and the lens 19 are arranged in a vicinity of the opening of the image sensor 13. In this case, the focal point of the image sensor 13 is adjusted by the lens 19 to the sheet on the platen 17.
At the time of the printing, the thermal transfer film (ribbon) of the ribbon cassette 14 is interposed between the recording sheet that is transported on the platen 17 and the thermal head 12 and is pushed by the thermal head 12, so that ink of the thermal transfer film is transferred onto the recording sheet. On the other hand, at the time of the reading, the thermal head 12 is at a receded position with respect to the printed document that is transported, and the light source 18 of the image sensor 13 is turned ON so as to read the document by the solid state image pickup element 20 as the carriage 11 moves.
Although not shown in FIGS.1A and 1B, the sheet is transported in this serial printer 10 by transport rollers which are arranged in a vicinity of a supply side relative to the platen 17. Generally, no transport means is arranged on an eject side relative to the platen 17.
However, a gap on the order of 1 mm, for example, is inevitably formed between the platen 17 and the image sensor 13. As a result, the flapping or the like of the document occurs with respect to the document that is transported within the range of this gap, and a focal error, a change in the magnification or the like of the lens 19 are generated thereby. For this reason, there is a problem in that the quality of the reading deteriorates because a stepped part occurs at a connecting part particularly when reading an oblique line.
It is conceivable to increase the depth of field, however, the optical path will become long, and in this case, there is a problem in that the size of the serial printer becomes large.